Electrical submersible pumps for high volume wells such as oil wells normally have three major sections, a motor section and a pump section separated by a seal section. A shaft on the end of a rotor assembly of the motor extends through the seal section for rotating the centrifugal pump. The motor and the seal sections contain oil for lubricating and cooling.
The seal section has features to equalize the lubricating oil pressure with the well fluid pressure, and to accommodate thermal expansion of the lubrication oil. The pressure compensation is handled in various manners, all of which require the entry of well fluid into some component of the pump assembly. Often, the well fluid includes water with a high salt content, and thus a low resistance. The electrical motor may be damaged if the well fluid contacts the motor. Consequently, the prior art pressure compensation systems use various means to avoid entry of well fluid into the motor section.
Some types use a bellows or elastomeric barrier, such as shown in U.S. Pat. No. 3,947,709. Well fluid on the outside of the bellows acts against lubrication located on the inside of the bellows to equalize pressure and allow for temperature compensation.
Another prior art feature is the use of multiple seal chambers consisting of several partitions located within the seal section. Each partition has an aperture for the shaft to pass through and means for sealing the shaft so that fluid cannot leak around the shaft from one chamber to the other. It is necessary that the pressures in these seal chambers equal each other and approximately equal the well fluid pressure. To accomplish this in the prior art, U-tubes interconnect the chambers. The U-tubes are arranged so that they will allow lubricating oil to pass between chambers in both directions, but discourage the heavier well fluids from migrating between the chambers. In this prior art system a liquid interface always exists between the chambers.